First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility

A. Adamczak; D. Bakalov; G. Baldazzi; M. Baruzzo; R. Benocci; R. Bertoni; M. Bonesini; S. Capra; D. Cirrincione; M. Clemenza; L. Colace; M. Danailov; P. Danev; A. de Bari; C. De Vecchi; D. Di Ferdinando; E. Fasci; R. Gaigher; L. Gianfrani; A. D. Hillier; K. Ishida; J. S. Lord; A. Menegolli; E. Mocchiutti; S. Monzani; L. Moretti; G. Morgante; C. Pizzolotto; A. Pullia; M. Pullia; R. Ramponi; H. E. Roman; M. Rossella; R. Rossini; A. Sbrizzi; M. Stoilov; J. J. Suárez-Vargas; G. Toci; L. Tortora; E. Vallazza; K. Yokoyama; A. Vacchi

doi:10.1140/epja/s10050-025-01758-4

2024 Impact factor 2.8

Hadrons and Nuclei

Open Access

Eur. Phys. J. A (2025) 61: 284
https://doi.org/10.1140/epja/s10050-025-01758-4

Regular Article - Experimental Physics

First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility

A. Adamczak¹, D. Bakalov², G. Baldazzi³^,4, M. Baruzzo⁵, R. Benocci⁶^,7, R. Bertoni⁶, M. Bonesini⁶^,8, S. Capra⁹^,10, D. Cirrincione⁵, M. Clemenza⁶^,8, L. Colace¹¹^,12, M. Danailov⁵^,13, P. Danev², A. de Bari¹⁴^,15, C. De Vecchi¹⁵, D. Di Ferdinando⁴, E. Fasci¹⁶^,17, R. Gaigher⁶, L. Gianfrani¹⁶^,17, A. D. Hillier¹⁸, K. Ishida¹⁹, J. S. Lord¹⁸, A. Menegolli¹⁴^,15^a, E. Mocchiutti⁵, S. Monzani⁵^,20, L. Moretti¹⁶^,17, G. Morgante³^,21, C. Pizzolotto⁵, A. Pullia⁹^,10, M. Pullia¹⁵^,22, R. Ramponi⁹^,23, H. E. Roman⁶^,8, M. Rossella¹⁵, R. Rossini¹⁴^,15, A. Sbrizzi³^,4, M. Stoilov², J. J. Suárez-Vargas⁵^,25^,25, G. Toci²⁴, L. Tortora¹¹, E. Vallazza⁶, K. Yokoyama¹⁸ and A. Vacchi⁵^,19^,20

¹ Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31342, Kraków, Poland
² Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, blvd. Tsarigradsko ch. 72, 1784, Sofia, Bulgaria
³ Dipartimento di Fisica e Astronomia “A. Righi”, Università di Bologna, viale Berti Pichat 6/2, Bologna, Italy
⁴ Sezione INFN di Bologna, viale Berti Pichat 6/2, Bologna, Italy
⁵ Sezione INFN di Trieste, via A. Valerio 2, Trieste, Italy
⁶ Sezione INFN di Milano Bicocca, Piazza della Scienza 3, Milano, Italy
⁷ Dipartimento di Scienze dell’Ambiente e della Terra, Università di Milano Bicocca, Piazza della Scienza 1, Milano, Italy
⁸ Dipartimento di Fisica “G. Occhialini”, Università di Milano Bicocca, Piazza della Scienza 3, Milano, Italy
⁹ Sezione INFN di Milano, via Celoria 16, Milano, Italy
¹⁰ Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, Milano, Italy
¹¹ Sezione INFN di Roma Tre, Via della Vasca Navale 84, Rome, Italy
¹² Dipartimento di Ingegneria, Università degli Studi Roma Tre, Via V. Volterra 62, Rome, Italy
¹³ Sincrotrone Elettra Trieste, SS14, km 163.5, Basovizza, Italy
¹⁴ Dipartimento di Fisica “A. Volta”, Universitá di Pavia, via A. Bassi 6, Pavia, Italy
¹⁵ Sezione INFN di Pavia, Via A. Bassi 6, Pavia, Italy
¹⁶ Sezione INFN di Napoli, Via Vicinale Cupa Cintia 26, Napoli, Italy
¹⁷ Dipartimento di Matematica e Fisica “L. Vanvitelli”, Università della Campania, Viale Lincoln 5, Caserta, Italy
¹⁸ ISIS Neutron and Muon Source, STFC Rutherford-Appleton Laboratory, OX11 0QX, Didcot, UK
¹⁹ Riken Nishina Center, RIKEN, 2-1 Hirosawa, 351-0198, Wako, Saitama, Japan
²⁰ Dipartimento di Scienze Matematiche, Informatiche e Fisiche, Università di Udine, via delle Scienze 206, Udine, Italy
²¹ INAF-OAS Bologna, via P. Gobetti 93/3, Bologna, Italy
²² Centro Nazionale di Adroterapia Oncologica (CNAO), Via Borloni 1, Pavia, Italy
²³ IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, Milano, Italy
²⁴ INO-CNR, via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy
²⁵ INFN LNS, Catania, Italy

^a alessandro.menegolli@unipv.it

Received: 15 September 2025
Accepted: 22 November 2025
Published online: 24 December 2025

Abstract

The FAMU experiment, supported and funded by the Italian Institute of Nuclear Physics (INFN) and by the Science and Technology Facilities Council (STFC), aims to perform the first measurement of the ground-state hyperfine splitting (1S-hfs) of muonic hydrogen ( $μ H$ $\mu H$ ). This quantity is highly sensitive to the proton’s Zemach radius $R_{Z}$ $$R_Z$$ . An experimental determination of $R_{Z}$ $$R_Z$$ provides significant constraints on the parametrisation of the proton form factors as well as on theoretical models describing the proton’s electromagnetic structure. Following years of technological and methodological development, the FAMU experiment began operations in 2023 at Port 1 of the RIKEN-RAL muon beam line at the ISIS Neutron and Muon Source facility (Didcot, UK). In this paper, we first describe the unique detection technique employed by FAMU to determine the 1S-hfs of muonic hydrogen, followed by a detailed presentation of the final experimental layout. Finally, we report the first outcome from the 2023 commissioning run and from the initial physics runs performed in 2023 and 2024.

Communicated by Klaus Peters.

Present address: INFN LNS, Catania, Italy

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First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility

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